Active Freeze Drying - another Hosokawa innovation

Freeze-drying (or lyophilization) is the sublimation of a solvent, which has been crystallised at low temperatures. The solvent is sublimated from the solid state direct into the vapour phase. By freeze drying a product can be obtained with good shelf stability and which will remain unchanged after reconstitution with the solvent. In other words by the freeze drying process the material to be dried is frozen in its original structure and the solvent is removed from this structure without changing this structure.

Why Active Freeze Drying

Until now, freeze dryers are all of the tray dryer type. Despite the optimal structure of the individual product particles the product layer on the plates will usually form one piece of hard baked material. Often the product has to be crushed after freeze drying, which may lead to damage to the product structure. Another disadvantage could be the relative low heat transfer rate due to the quiescent state of the material. Also the handling of the product is quite labour intensive because the trays have to be filled and discharged manually.

Experiments with an agitated vacuum dryer operated at low temperature and low pressure showed that it is possible to produce a lump-free, free-flowing, freeze dried product. Active freeze dryers also have a better heat transfer rate due to the continuous mixing of the product, thus shortening the drying process. Finally the freezing step is simplified because it can be done in the same vessel instead of having a separate freezing unit, which results in easy handling of the product compared to tray drying equipment.

Recent experiments also indicate that the active freeze dryer can be used in continuous operation. Since the dried product is released from the frozen material during the drying process, these dry particles can be collected in a filter next to the actual dryer.

Applications of Active Freeze Dryers

Typical applications of this new technology are to be expected in e.g. the production of pro-biotics (bacteria as food additives), pharmaceuticals and new (nano) materials.